by PhilipJ on 7 January 2010

Ribosomes are one of the wonders of the cellular world, and one of the many wonders you can explore yourself at the RCSBPDB. In 2000, structural biologists Venkatraman Ramakrishnan, Thomas A. Steitz and Ada E. Yonath made the first structures of ribosomal subunits available in the PDB, and in 2009, they each received a Nobel Prize for this work. Structures are also available for many of the other players in protein synthesis, including transfer RNA and elongation factors. Building on these structures, there are now hundreds of structures of entire ribosomes in the PDB, revealing the atomic details of many important steps in protein synthesis.

After solving the structures of the individual small and large subunits, the next step in ribosome structure research was to determine the structure of the whole ribosome. This work is the culmination of decades of research, which started with blurry pictures of the ribosome from electron microscopy, continued with more detailed cryoelectron micrographic reconstructions, and now includes many atomic structures. By using small pieces of mRNA, various forms of shortened or chemically-modified tRNA, purified protein factors, and modified ribosomes, researchers have solved the structures of ribosomes in the act of building proteins. These structures are so large that they don’t fit into a single PDB file—for instance, the structure shown here was split into PDB entries 2wdk and 2wdl.

Looking at all the different forms of life on the Earth, we find that all living organisms have ribosomes and that they come in two basic sizes. Bacteria and archaebacteria have smaller ribosomes, termed 70S ribosomes, which are composed of a small 30S subunit and large 50S subunit. The “S” stands for svedbergs, a unit used to measure how fast molecules move in a centrifuge (note that the values for the individual subunits don’t add up to the value for the whole ribosome, since the rate of sedimentation is related in a complex way to the mass and shape of the molecule). The ribosomes in our cells, and in other animals, plants and fungi, are larger, termed 80S ribosomes, composed of a 40S small subunit and a 60S large subunit. Strangely, our mitochondria have small 70S ribosomes that are made separately from the larger ones in the cytoplasm. This observation has lead to the hypothesis that mitochondria (and chloroplasts in plant cells) are actually bacteria that were caught inside cells early in the evolution of eukaryotic cells. Now, they live and reproduce happily inside cells, focusing on energy production and relying on the surrounding cell for most of their other needs.

As I biology tutor I use the edosymbiotic theory to explain the difference in size between a prokaryotic cell and a eukaryotic cell. Now I have something more to share with my students. Thanks for the post.